Transcription of many plant genes is controlled in a temporal and spatial manner. The regulation of gene activity is mediated by the interaction of trans acting factors and cis regulatory elements in the promoter region of a gene. Recent work has elucidated the working of light-regulated genes in plants as well as organ-specific expression and developmentally controlled abundant gene products such as seed storage proteins. Benfey et al., Science 244: 174-181 (1989). For example, Barker et al., PNAS (USA) 85: 458-462 (1988) have transformed a gene encoding a major seed storage protein from soybean into tobacco and have shown the protein to be expressed in the proper temporal and developmental patterns. Fluhr et al., Science 232: 1106-1112 (1986) showed a 5'-fragment from a pea rbcS gene to be responsible for leaf-specificity as well as light response in that gene.
Colot et al., EMBO 6:3559-3563 (1987) described promoter sequences from wheat endosperm protein genes that direct a tissue-specific expression pattern in transgenic tobacco similar to that seen in wheat.
It has been suggested that promoters may contain several active sub-elements, or domains, that confer some differential expression properties. For example, much work has been done with the cauliflower mosaic virus (CaMV) promoter 35S. Lam et al., The Plant Cell, 1: 1147-1156 (1989) have shown that the CaMV 35S promoter consists of at least two domains; Domain A confers preferential expression in roots; Domain B confers preferential expression in leaf. When Domain A was added to the pea rbcS3A promoter, which is a green tissue specific promoter, the resulting construct promoted expression in roots. In seeds, expression from domain A was detected in the radicle of the embryo and expression from domain B was detected primarily in the cotyledons. Lam et al., PNAS USA, 86: 7890-7894 (1989) found that the ASF-1 binding site of the CaMV 35S promoter is required for high expression of the 35S promoter in the root.
Inducible gene activity has been studied in various systems and promoter analysis has identified regions involved in the inducible control of gene activity in these systems. One example of a class of inducible genes is the animal metallothionein protein genes. Expression of mammalian metallothionein protein genes are induced by the presence of elevated concentrations of trace metals, hormones and stress. Palmiter, Metallothionein II, 63-80 (ed. Kagi et al. Binkhauser, Verlag, Basel 1987). It is also known that various plant genes are inducible by chemical regulators. For example, the production of chitinase is induced by ethylene. Boller et al., Planta, 157:22-31 (1983).
Despite their important role in plant development, relatively little work has been done on the regulation of gene expression in roots. Yamamoto, A Tobacco Root-Specific Gene; Characterization and Regulation of its Transcription, (Thesis North Carolina State University Genetics Department, 1989), reported the isolation of genes that are expressed at high levels in tobacco roots and undetectable levels in tobacco leaves. 5' flanking regions from one such gene were fused to a reporter gene. Root specific expression of the fusion genes was analyzed in transgenic tobacco. Yamamoto further characterized one of those genes, the TobRB7-5A gene, including the promoter region. Yamamoto theorized that the gene may contain generalized transcriptional enhancers, or additional root-specific elements.